1. Field of the Invention
This invention relates to a bicycle gear crank assembly which is rotated by a rider to drive the bicycle.
2. Description of the Prior Art
A gear crank assembly for a bicycle consists of a crank having a boss portion fixed to one end of a crank shaft, and a chainwheel (front gear) mounted to the crank boss portion for rotation therewith. The crank of the gear crank assembly forms a pair with another crank fixed to the other end of the crank shaft, whereas the chainwheel is connected to a freewheel (rear gear) by means of a chain. Thus, when the paired cranks are rotated by the rider, the chainwheel is rotated together with the freewheel to drivingly rotate the rear wheel of the bicycle.
In actual running, the crank and chainwheel of the gear crank assembly are frequently subjected to a large pedalling force particularly at the time of accelerating and/or running on an upward slope. Thus, the chainwheel, which itself is mechanically strong enough, must be mounted to the crank in such a way as to withstand any pedalling force.
In a most typical form, the chainwheel is made to have a central mounting bore which is serrated, and the boss portion of the crank is externally serrated for non-rotatably press-fitting in the mounting bore of the chainwheel. To complete the assembly, the crank boss portion is subjected to calking to axially retain the chainwheel relative to the crank boss portion. Obviously, this manner of mounting can be performed very easily because there is no need to conduct bolting the chainwheel to the crank boss portion.
The typical form of gear crank assembly is acceptable as long as the chainwheel includes only a single sprocket. However, the typical form of gear crank assembly becomes problematic if the chainwheel is made to have two or more diametrically different sprockets in an attempt to increase the number of speeds obtainable in combination with a multiple freewheel. The problem is now described.
As is well known, in shifting the chain from one sprocket to another of a multiple chainwheel, the chain is laterally displaced by a front derailleur relative to the chainwheel. Such chain shifting occurs relatively easily when shifting from a largest sprocket to a smaller sprocket because the chain needs only to fall down to the smaller sprocket without laterally pressing against the smaller sprocket. However, when shifting the chain from the smaller sprocket to the largest sprocket, the chain must laterally press the largest sprocket before completely shifting thereto. Such lateral pressing may cause the largest sprocket to bend laterally away from the smaller sprocket, thus hindering smooth chain shifting in addition to damaging the chain or the teeth of the largest sprocket
The lateral bending of the largest sprocket may be prevented by enlarging the wall thickness of that sprocket. However, such a solution cannot be readily adopted because the wall thickness of the largest sprocket (and other sprockets as well) is inevitably limited by the wanted overall weight and by the dimensions of the chain.
U.S. Pat. No. 4,259,880 to Ueno discloses a gear crank assembly which comprises a crank having a boss portion formed with a plurality of integral sprocket mounting arms extending radially outward from the boss portion. The assembly further comprises a multiple chainwheel including two diametrically different sprockets fixedly mounted to the sprocket mounting arms by means of bolting.
According to the arrangement of Ueno, the larger (largest) sprocket of the chainwheel, instead of being directly mounted to the crank boss portion, is mounted to the sprocket mounting arms which has a substantial radial length. Thus, the radial length (from the tooth tips to the bolt connection) of the larger sprocket itself can be rendered correspondingly smaller, so that the larger sprocket is unlikely to bend laterally even if a large bending force is applied thereto.
However, the arrangement proposed by Ueno requires bolting operation for mounting the larger sprocket to the sprocket mounting arms of the crank boss portion, consequently requiring a longer time for mounting than simply fitting a sprocket. Further, the configuration of the larger sprocket departs very greatly from a typical sprocket configuration, consequently resulting in a cost increase for production. Thus, the arrangement of Ueno is acceptable only for high grade bicycles wherein more importance is placed on the performance than on cost.